In recent years, particulates such as soot, contained in exhaust gases that are discharged from internal combustion engines of vehicles such as buses and trucks and construction equipment, have raised serious problems as contaminants harmful to the environment and the human body.
For this reason, various honeycomb structural bodies, made from porous ceramics, have been proposed as filters capable of collecting particulates from exhaust gases to purify the exhaust gases.
Conventionally, with respect to the honeycomb structural bodies of this type, filters have been proposed (for example, see Japanese Patent gazette No. 3130587 and FIG. 17 of U.S. Pat. No. 4,417,908) in which through holes of two kinds, that is, a through hole with a larger capacity (hereinafter, also referred to as large-capacity through hole) and a through hole with a smaller capacity (hereinafter, also referred to as small-capacity through hole) are formed, and the end of the large-capacity through hole on the exhaust gas outlet side is sealed with a sealing material with the end of the small-capacity through hole on the exhaust gas inlet side being sealed with a sealing member, so that the surface area of a through hole with the opened inlet side (hereinafter, also referred to as inlet-side through hole) is made relatively larger than the surface area of a through hole with the opened outlet side (hereinafter, also referred to as outlet-side through hole); thus, the filters make it possible to increase the collection amount of particulates, to lengthen the period up to the recovery process and also to achieve a small-sized device.
Moreover, filters have been disclosed (for example, see FIG. 3 of U.S. Pat. No. 4,417,908) in which the number of inlet-side through holes is made greater than the number of outlet-side through holes, with the surface area of the inlet-side through holes being also made relatively greater than the surface area of the outlet-side through holes; thus, the filters make it possible to increase the collection amount of particulates, to lengthen the period up to the recovery process and also to achieve a small-sized device.
In other words, in the honeycomb structural bodies to be used as exhaust gas purifying filters that are disclosed in Japanese Patent gazette No. 3130587 as well as in U.S. Pat. No. 4,417,908, in comparison with a honeycomb structural body with the total surface area of the inlet-side through holes being set to the same as the total surface area of the outlet-side through holes, the total surface area of the inlet-side through holes is made relatively greater so that it becomes possible to make the thickness of a deposition layer for collected particulates thinner; consequently, it becomes possible to increase the limiting collection amount of particulates as described above.
Additionally, such conventional techniques include those described in the following patent literatures.
JP Kokai Sho 56-124418 has disclosed a ceramic filter in which through holes having shapes, such as a triangle, a hexagonal shape, a round shape and a convex shape, are formed. Moreover, U.S. Pat. No. 4,276,071 (FIGS. 5a to 5p), JP Kokai Sho 56-124417, JP Kokai Sho 62-96717 and U.S. Pat. No. 4,364,761 (FIGS. 5a to 5p) have disclosed arrangements similar to that of JP Kokai Sho 56-124418.
Microfilms of Japanese Utility Model Application No. 56-187890 (J UM Kokai Sho 58-92409 (FIG. 6, page 4) have disclosed an exhaust gas filter in which triangular through holes and hexagonal through holes are formed with cell pitches of large-capacity through holes being set approximately in a range from 1.0 to 2.5 mm.
U.S. Pat. No. 4,416,676 (FIGS. 1 to 4) has disclosed a honeycomb filter in which through holes having shapes, such as a triangle, a square, an octagonal shape and a round shape, are formed with the relationship between the wall thickness between large-capacity through holes and the wall thickness between the large-capacity through hole and the small-capacity through hole is defined.
JP Kokai Sho 58-196820, JP Kokoku Hei 3-49608 and U.S. Pat. No. 4,417,908 (FIGS. 3 to 17) have disclosed honeycomb filters in which through holes having shapes such as a triangle, a square and a hexagonal shape as well as honeycomb filters in which the number of through holes on the inlet side is made greater than the number of through holes on the outlet side so that the aperture rate on the exhaust gas inlet side is made relatively greater than the aperture rate on the exhaust gas outlet side.
U.S. Pat. No. 4,420,316 (FIGS. 6 to 9) has disclosed a honeycomb filter in which the number of sealed through holes is modified, which relates to a technique for improving the gas flow rate in the wall portion.
JP Kokai Sho 58-150015 has disclosed a filter which is provided with square through holes and rectangular through holes, with the cross-sectional shape of the through holes being formed into a tapered shape so as to be changed from the gas inlet side toward the outlet side.
JP Kokai Hei 5-68828 and Japanese Patent gazette No. 3130587 (page 1) have disclosed honeycomb filters in which triangular through holes and hexagonal through holes are formed and the capacity rate of the large-capacity through holes is set to 60 to 70% while the capacity rate of the small-capacity through holes is set to 20 to 30%, with the cell pitch of the large-capacity through holes being set to approximately in a range from 2.5 to 5.0 mm.
French Patent No. 2789327 has disclosed a filter that is provided with through holes having shapes such as a rectangular shape, a square shape, a hexagonal shape and an octagonal shape, with the cross-sectional shape of the through holes being formed into a tapered shape so as to be changed from the gas inlet side toward the outlet side.
International Publication No. 02/100514 and JP Kokai 2001-334114 (FIG. 2) have disclosed filters in which through holes having a round shape and a hexagonal shape are formed. These have also disclosed filter elements in which the ratio of the total area of the cross-section of small-capacity through holes to the total area of the cross-section of large-capacity through holes is set in a range from 40 to 120%.
International Publication No. 02/10562 has disclosed a filter in which square through holes and hexagonal through holes are formed, with the ratio of cross-sections thereof being set in a range from 3:1 to 4:1.
International Publication No. 03/20407 has disclosed a honeycomb structural body in which square through holes are formed with a varied ratio of cross-sectional areas.
Moreover, in addition to the above-mentioned honeycomb structural bodies each of which is formed into an integral structure as a whole (hereinafter, also referred to as integrated filter), JP Kokai 2001-162121 has disclosed a honeycomb structural body in which a plurality of columnar ceramic members are combined with one another through seal material layers (hereinafter, referred to as aggregate-type filter).
However, in the case where the integrated filter is formed into a column shape, cracks tend to occur at corner portions (four corners) due to a thermal stress imposed thereon upon burning particulates. In the same manner, at corner portions (four corners) of columnar ceramic members forming an aggregate-type filter, cracks also tend to occur due to a thermal stress imposed thereon upon burning particulates. Moreover, in the case where cells that have openings on the outlet side are located at corner portions (four corners), with through holes having different capacities being formed, as disclosed in U.S. Pat. No. 4,417,908, the occurrence of cracks becomes more conspicuous.